Fluid systems, such as cooling, heating, petroleum refining, pneumatic or other vapor or gas system, waste water control, or chemical process systems, typically utilize valves to control fluid flow. These fluid control devices may include a variety of different types, sizes, and configurations of valves, such as globe valves, ball valves, butterfly valves, and plug valves.
Several factors affect the design of fluid control valves. As used herein fluid may encompass, liquid, vapor, gas or a mixture of any of these fluid phases. The type of fluid that is regulated by the valve may influence the materials and dimensions of the valve components. For example, some gasoline refining applications may require valves to control the flow of a high-temperature fluid including crude oil and erosive particulates, such as dirt and/or certain catalytic agents. As this erosive fluid flows through the valve, the components may be subjected to temperatures in excess of 500° Fahrenheit and in extreme cases in excess of 1000° Fahrenheit and pressure differential across the valve greater than 3000 psi. High pressure drops in the flowing liquid may result in cavitation, which may damage valve components. In pipeline service, valves may be subjected to high pressure differentials across valve seats that result in high closing forces placed on a valve actuator when opening and closing the valve. These are some of the factors that should be considered when selecting a type of valve for a particular type of service.
External factors, such as noise attenuation, may also affect the choice of fluid control valves. For example, design considerations may indicate that a control valve operating in an expanding flow direction such that the fluid flows in a radially outward direction passing through the cage (e.g., flow moving upward, through the cage aid expanding toward a greater volume in the valve gallery) is preferable. Such valves, for instance, may be desirable for compressible flow applications where flow noise minimization is important. Further, design considerations may indicate that a pilot plug operated control valve may be preferable. Pilot plug operated control valves, however, may often be limited to a downward flow design where the control valve operates in a contracting flow direction such that the fluid flows in a radially inward direction passing through the cage. Additionally, in some instances, design considerations may indicate that a pressure balanced pilot plug operated control valve (e.g., pressure on both sides of the plug is the same when valve is closed) may be preferable rather than a pressure unbalanced valve (e.g., pressure is different on either side of the plug when valve is closed).
Currently, a variety of seals are available to produce a balanced valve. Such seals may be utilized between a plug and a cage of a control valve. For instance, piston ring seals between the cage and plug may be manufactured from multiple distinct materials depending on the valve application (i.e., type of fluid, temperature, pressure). Teflon, metal, and graphite are just a handful of examples of materials from which such piston ring seals may be made. Teflon piston ring seals, for instance, may allow for a reasonably tight shut-off but be limited in usage by fluid temperature. Graphite and metal piston ring seals may allow for the valve to be used in higher temperature applications, but such materials may not allow for tight shut-off (e.g., ANSI FCI 70-2 Class IV or lower).
Balanced valves may also utilize pressure energized seals, which may allow flow in both directions with a reversible seal. Such seals may generally be manufactured of engineered plastics and can produce excellent shut-off characteristics (e.g., ANSI Class V). Valves utilizing pressure energized seals, however, may be temperature limited in their applicability.
Deformable metal seals may also provide for a pressure balanced control valve. Deformable metal sealing, typically, may allow for a tight shut-off by forming a metal-to-metal seal between the plug and the cage. Further, such seals may allow for a valve to be utilized in high temperature applications. A tight shut-off through deformable metal seals, however, may be dependent on precise manufacturing tolerances, thus increasing the cost of valve production. Additionally, deformable metal seals may deteriorate over time.
Accordingly, certain fluid regulation applications and systems may be best suited for a control valve operable in an expanding flow direction, while maintaining tight shut-off capability and a wide range of temperature and pressure applicability.